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Subject: Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Yu Zhao <yuzhao@google.com>,
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        Peter Xu <peterx@redhat.com>,
        Pavel Emelyanov <xemul@openvz.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Minchan Kim <minchan@kernel.org>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Leon Romanovsky <leonro@nvidia.com>, Jan Kara <jack@suse.cz>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Nadav Amit <nadav.amit@gmail.com>, Jens Axboe <axboe@kernel.dk>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <a21e6fdf-5cac-6fda-242e-7909af96027a@redhat.com>
Date:   Fri, 15 Jan 2021 20:46:48 +0100
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>> 7) There is no easy way to detect if a page really was pinned: we might
>> have false positives. Further, there is no way to distinguish if it was
>> pinned with FOLL_WRITE or not (R vs R/W). To perform reliable tracking
>> we most probably would need more counters, which we cannot fit into
>> struct page. (AFAIU, for huge pages it's easier).
> 
> I think this is the real issue. We can only store so much information,
> so we have to decide which things work and which things are broken. So
> far someone hasn't presented a way to record everything at least..

I do wonder how many (especially long-term) GUP readers/writers we have
to expect, and especially, support for a single base page. Do we have a
rough estimate?

With RDMA, I would assume we only need a single one (e.g., once RDMA
device; I'm pretty sure I'm wrong, sounds too easy).
With VFIO I guess we need one for each VFIO container (~ in the worst
case one for each passthrough device).
With direct I/O, vmsplice and other GUP users ?? No idea.

If we could somehow put a limit on the #GUP we support, and fail further
GUP (e.g., -EAGAIN?) once a limit is reached, we could partition the
refcount into something like (assume max #15 GUP READ and #15 GUP R/W,
which is most probably a horribly bad choice)

[ GUP READ ][ GUP R/W ] [  ordinary ]
31  ...  28 27  ...  24 23   ....   0

But due to saturate handling in "ordinary", we would lose further 2 bits
(AFAIU), leaving us "only" 22 bits for "ordinary". Now, I have no idea
how many bits we actually need in practice.

Maybe we need less for GUP READ, because most users want GUP R/W? No idea.

Just wild ideas. Most probably that has already been discussed, and most
probably people figured that it's impossible :)

-- 
Thanks,

David / dhildenb